WEBDAV Working Group J. Slein, Xerox INTERNET DRAFT E.J. Whitehead Jr., UC Irvine J. Davis, CourseNet G. Clemm, Rational C. Fay, FileNet J. Crawford, IBM T. Chihaya, DataChannel August 20, 1999 Expires February 20, 2000 WebDAV Bindings Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Distribution of this document is unlimited. Please send comments to the Distributed Authoring and Versioning (WebDAV) working group at , which may be joined by sending a message with subject "subscribe" to . Discussions of the WEBDAV working group are archived at URL: . Abstract The WebDAV Distributed Authoring Protocol provides basic support for collections, offering the ability to create and list unordered collections. This specification is one of a group of three specifications that supplement the WebDAV Distributed Authoring Protocol to increase the power of WebDAV collections. This specification defines bindings, one mechanism for allowing a single resource to appear in more than one collection. Bindings make this possible by creating mappings of URIs to resources. The BIND method defined here gives clients the ability to create new bindings to existing resources. [RR] defines redirect references, another approach to allowing a single resource to be accessed from multiple collections. [OC] provides ordered collections. Table of Contents 1 Notational Conventions.......................................2 2 Introduction.................................................2 3 Terminology..................................................4 Slein et al. Page 1 Internet-Draft WebDAV Bindings August 1999 4 Overview of Bindings.........................................7 5 BIND Method..................................................8 5.1 Overview of BIND.............................................8 5.2 Bindings to Collections......................................9 5.3 URI Mappings Created by BIND.................................9 5.4 Example: Generating the Set of URI Mappings.................10 5.5 BIND Status Codes...........................................11 5.6 Example: BIND...............................................11 5.7 Example: BIND Conflict......................................11 6 DELETE and Bindings.........................................12 7 COPY and Bindings...........................................12 8 MOVE and Bindings...........................................13 8.1 Implementation Note.........................................14 9 LOCK and UNLOCK.............................................15 10 Bindings and Other Methods..................................15 11 Discovering the Bindings to a Resource......................16 12 Headers.....................................................16 12.1 All-Bindings Request Header.................................17 13 Status Codes................................................17 13.1 506 Loop Detected...........................................17 14 Properties..................................................17 14.1 bindings Property...........................................17 15 XML Elements................................................17 15.1 segment XML Element.........................................17 16 Capability Discovery........................................17 16.1 Example: Discovery of Support for Bindings..................18 17 Security Considerations.....................................18 17.1 Privacy Concerns............................................18 17.2 Redirect Loops..............................................19 17.3 Bindings, and Denial of Service.............................19 17.4 Private Locations May Be Revealed...........................19 17.5 DAV:bindings and Denial of Service..........................19 18 Internationalization Considerations.........................19 19 IANA Considerations.........................................20 20 Copyright...................................................20 21 Intellectual Property.......................................20 22 Acknowledgements............................................20 23 References..................................................20 24 Authors' Addresses..........................................21 25 Appendices..................................................21 25.1 Appendix 1: Extensions to the WebDAV Document Type Definition..................................................21 1 Notational Conventions Since this document describes a set of extensions to the HTTP/1.1 protocol, the augmented BNF used here to describe protocol elements is exactly the same as described in Section 2.1 of [HTTP]. Since this augmented BNF uses the basic production rules provided in Section 2.2 of [HTTP], these rules apply to this document as well. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2 Introduction Slein et al. Page 2 Internet-Draft WebDAV Bindings August 1999 The simple collections that the WebDAV Distributed Authoring Protocol specification supports are powerful enough to be widely useful. They provide for the hierarchical organization of resources, with mechanisms for creating and deleting collections, copying and moving them, locking them, adding members to them and removing members from them, and getting listings of their members. Delete, copy, move, list, and lock operations can be applied recursively, so that a client can operate on whole hierarchies with a single request. This specification is one of a family of three specifications that build on the infrastructure defined in [HTTP] and [WebDAV] to extend the capabilities of collections. The companion specification [OC] defines protocol extensions to support ordered collections. The present specification and the companion specification [RR] define mechanisms for allowing the same resource to appear in multiple collections. This capability is useful for several reasons: Organizing resources into hierarchies places them into smaller groupings, known as collections, which are more easily browsed and manipulated than a flat namespace. However, hierarchies require categorization decisions that locate resources at a single location in the hierarchy, a drawback when a resource has multiple valid categories. For example, in a hierarchy of vehicle descriptions containing collections for cars and boats, a description of a combination car/boat vehicle could belong in either collection. Ideally, the description should be accessible from both. Hierarchies also make resource sharing more difficult, since resources that have utility across many collections are still forced into a single collection. For example, the mathematics department at one university might create a collection of information on fractals that contains bindings to some local resources, but also provides access to some resources at other universities. For many reasons, it may be undesirable to make physical copies of the shared resources on the local server - to conserve disk space, to respect copyright constraints, or to make any changes in the shared resources visible automatically. The companion specification [RR] defines redirect references, one mechanism for providing access to a single resource from multiple collections. A redirect reference is a resource in one collection whose purpose is to forward requests to another resource (its target), usually in a different collection. In this way, it provides access to the target resource from another collection. It redirects most requests to the target resource using the HTTP 302 (Moved Temporarily) status code, thereby providing a form of mediated access to the target resource. The BIND method defined here provides a different mechanism for allowing a single resource to appear in multiple collections. It lets clients associate a new URI with an existing resource. This URI can then be used to submit requests to the resource. Since URIs in WebDAV are hierarchical, and correspond to a hierarchy of collections in resource space, the BIND method also has the effect of adding the resource to a collection. As new URIs are associated with the resource, it appears in additional collections. Slein et al. Page 3 Internet-Draft WebDAV Bindings August 1999 These two approaches to allowing clients to add a single resource to multiple collections have very different characteristics: A redirect reference is a resource, and so can have properties of its own. Such information as who created the reference, when, and why can be stored on the redirect reference resource. Since redirect references are implemented using HTTP 302 responses, it generally takes two round trips to submit a request to the intended resource. Servers are not required to enforce the integrity of redirect references. Redirect references work equally well for local resources and for resources that reside on a different server from the reference. By contrast, a BIND request does not create a new resource, but simply makes available a new URI for submitting requests to an existing resource. The new URI can be used like any other URI to submit a request to a resource. Only one round trip is needed to submit a request to the intended target. Servers are required to enforce the integrity of the relationships between the new URIs clients create and the resources associated with them. Consequently, it is unlikely that servers will support BIND requests that cross server boundaries. 3 Terminology The terminology used here follows and extends that in the WebDAV Distributed Authoring Protocol specification [WebDAV]. Definitions of the terms resource, Uniform Resource Identifier (URI), and Uniform Resource Locator (URL) are provided in [URI]. URI Mapping A relation between an absolute URI and a resource. For an absolute URI U and the resource it identifies R, the URI mapping can be thought of as the ordered pair (U,R). Since a resource can represent items that are not network retrievable, as well as those that are, it is possible for a resource to have zero, one, or many URI mappings. Mapping a resource to an "http" scheme URL makes it possible to submit HTTP protocol requests to the resource using the URL. Path Segment Informally, the characters found between slashes ("/") in a URI. Formally, as defined in section 3.3 of [URI]. Binding A relation between a single path segment (in a collection) and a resource. A binding is part of the state of a collection. If two different collections contain a binding between the same path segment and the same resource, these are two distinct bindings. So for a collection C, a path segment S relative to that collection, and a resource R, the binding can be thought of as the ordered triple (C,S,R). Bindings create URI mappings, and hence allow requests to be sent to a single resource from multiple locations in a URI namespace. The following figure can be used to illustrate how bindings create Slein et al. Page 4 Internet-Draft WebDAV Bindings August 1999 URI mappings. +-----------------------------+ | root collection | |-----------------------------| | bindings: | | | | Coll1 Coll2 Coll3 | | | | \ | +---|-------|--------\--------+ | | \ | | \ +-------------------+ +----------------------+ | collection C1 | | collection C2 | |-------------------| |----------------------| | bindings: | | bindings: | | | | | | foo bar | | foo | | | \ | | / | +--|------\---------+ +-/--------------------+ | \ / | \ / | \ / | \ / +--------------+ +---------------+ | resource R1 | | resource R2 | +--------------+ +---------------+ Figure 1 The binding (C1,foo,R1) between foo and resource R1 in collection C1 creates the URI mappings /Coll1/foo and /Coll2/foo to resource R1. Each of these URI mappings can be used to submit requests to R1. The binding (C1,bar,R2) between bar and resource R2 in collection C1 and the binding (C2,foo,R2) between foo and resource R2 in collection C2 create altogether 3 URI mappings to resource R2: /Coll1/bar, /Coll2/bar, and /Coll3/foo. All 3 URI mappings can be used to submit requests to resource R2. Collection A resource that contains, as part of its state, a set of bindings that identify member resources. Internal Member URI The URI element U of a URI mapping (U,R), created by a binding that is contained in a collection. The following figure illustrates the relationship between bindings and internal member URIs in a collection: +-----------------------------+ | root collection | |-----------------------------| | internal member URIs: | | | | /Coll1/ | Slein et al. Page 5 Internet-Draft WebDAV Bindings August 1999 | /Coll2/ | | /Coll3/ | |-----------------------------| | bindings: | | | | Coll1 Coll2 Coll3 | | | | \ | +---|-------|--------\--------+ | | \ | | \ +----------------------+ +----------------------+ | collection C1 | | collection C2 | |----------------------| |----------------------| | internal member URIs:| | internal member URIs:| | | | | | /Coll1/foo | | /Coll3/foo | | /Coll2/foo | |----------------------| | /Coll1/bar | | bindings: | | /Coll2/bar | | | |----------------------| | foo | | bindings: | | / | | | +-/--------------------+ | foo bar | / | | \ | / +--|------\------------+ / | \ / | \ / | \ / | \ / +--------------+ +---------------+ | resource R1 | | resource R2 | +--------------+ +---------------+ Figure 2 The URI elements of all URI mappings created by a collection's bindings are internal member URIs of the collection. However, for a given request, only the URIs from those URI mappings that incorporate the Request-URI are treated as internal member URIs. For example, in Figure 2 above, if a PROPFIND request with "Depth: infinity" is submitted to collection C1 using the Request-URI /Coll1/, only the URI mappings starting with the Request-URI would be listed as internal member URIs. The response would include only /Coll1/ itself and the internal member URIs /Coll1/foo and /Coll1/bar. In [WebDAV], a collection is defined as containing a list of internal member URIs, where an internal member URI is the URI of the collection, plus a single path segment. This definition combines the two concepts of binding and URI mapping that are separated in this specification. As a result, this specification redefines a collection's state to be a set of bindings, and redefines an internal member URI to be the URI of a URI mapping derived from a binding. After this redefinition, an internal Slein et al. Page 6 Internet-Draft WebDAV Bindings August 1999 member URI can be used when reading [WebDAV] without loss of meaning. For purposes of interpretation, when [WebDAV] discusses a collection "containing" an internal member URI, this should be read as the collection containing a binding whose mapping to a URI creates an internal member URI, in this sense "containing" the internal member URI. The authors of this specification anticipate and recommend that future revisions of [WebDAV] perform a full reconciliation of terms between these two specifications. 4 Overview of Bindings Bindings are part of the state of a collection. In general, there is a one-to-many correspondence between a collection's bindings and its internal member URIs, as illustrated in Figure 2 above. The URI segment associated with a resource by one of a collection's bindings is also the final segment of one or more of the collection's internal member URIs. The final segment of each internal member URI identifies one of the bindings that is part of the collection's state, unless the internal member URI is not mapped to a resource. Even though a binding is just a relation between a path segment in a collection and a resource, a binding creates one or more URI mappings of a URI to a resource. For example, if the segment "foo.html" is being bound to a resource in a collection with URL "http://www.foo.net/A/", the binding creates a URI mapping of URL "http://www.foo.net/A/foo.html" to the HTML resource. If the parent collection is then bound to the segment "B", this creates two URI mappings, "http://www.foo.net/B/" to the collection resource, and "http://www.foo.net/B/foo.html" to the HTML resource. Both the collection and the HTML resource are now accessible via two URLs apiece. For a resource implemented by a computer, the relationship between a URI mapping and a resource is highlighted in Figure 1: URI 1 URI 2 ... URI N | | | | | | <------- URI Mappings | | | +---------------------+ | Resource R | +---------------------+ Figure 3 This resource can have multiple URIs mapped to it. Bindings are not unique to advanced collections, although the BIND method for explicitly creating bindings is introduced here. Existing methods that create resources, such as PUT, MOVE, COPY, and MKCOL, implicitly create bindings. There is no difference between implicitly created bindings and bindings created with BIND. The identity of a binding (C,S,R) is determined by the URI segment (in its collection) and the resource that the binding associates. If the resource goes out of existence (as a result of some out-of-band Slein et al. Page 7 Internet-Draft WebDAV Bindings August 1999 operation), the binding also goes out of existence. If the URI segment comes to be associated with a different resource, the original binding ceases to exist and another binding is created. Since a binding is a relation between a path segment in a collection and a resource, it would be very undesirable if one binding could be destroyed as a side effect of operating on the resource through a different binding. It is not acceptable for a MOVE through one binding to fail to update another binding, turning that binding into a dangling path segment. Nor is it acceptable for a server, after performing a DELETE through one binding, to reclaim the system resources associated with its resource while other bindings to the resource remain. Implementations MUST act to ensure the integrity of bindings. It is especially difficult to maintain the integrity of cross-server bindings. Unless the server where the resource resides knows about all bindings on all servers to that resource, it may unwittingly destroy the resource or move it without notifying another server that manages a binding to the resource. For example, if server A permits creation of a binding to a resource on server B, server A must notify server B about its binding and must have an agreement with B that B will not destroy the resource while A's binding exists. Otherwise server B may receive a DELETE request that it thinks removes the last binding to the resource and destroy the resource while A's binding still exists. Consequently, support for cross-server bindings is OPTIONAL. 5 BIND Method 5.1 Overview of BIND The BIND method creates a new binding from the final segment of the Request-URI (minus any trailing slash) to the resource identified by the Destination header. This binding is added to the collection identified by the Request-URI minus its trailing slash (if present) and final segment. The Destination header is defined in Section 9.3 of [WebDAV]. If a server cannot guarantee the binding behavior specified for GET (Section 10), DELETE (Section 6), and MOVE (Section 8), the BIND request MUST fail with a 501 (Not Implemented) status code. If the Request-URI ends in a slash ("/") (i.e., the Request-URI identifies a collection), the resource identified by the Destination header MUST be a collection resource, or the request fails with a 409 (Conflict) status code. This ensures that URIs ending in a slash are always bound to collections. If the Request-URI does not contain a path segment (i.e., it consists simply of a slash "/"), the BIND operation MUST fail and report a 409 (Conflict) status code. After successful processing of a BIND request, it MUST be possible for clients to use the Request-URI to submit requests to the resource identified by the Destination header. By default, if the Request-URI identifies an existing binding, the new Slein et al. Page 8 Internet-Draft WebDAV Bindings August 1999 binding replaces the existing binding. This default binding replacement behavior can be overridden using the Overwrite header defined in Section 9.6 of [WebDAV]. 5.2 Bindings to Collections BIND can create a binding to a collection resource. A collection accessed through such a binding behaves exactly as would a collection accessed through any other binding. Bindings to collections can result in loops, which servers MUST detect when processing "Depth: infinity" requests. When a loop is detected, the server MUST respond with a 506 (Loop Detected) status code (defined in Section 13.1). Creating a new binding to a collection makes each resource associated with a binding in that collection accessible via a new URI, but does not result in the creation of a new binding for each of these resources. For example, suppose a new binding COLLX is created for collection C1 in the figure below. It immediately becomes possible to access resource R1 using the URI /COLLX/x.gif and to access resource R2 using the URI /COLLX/y.jpg, but no new bindings for these child resources were created. This is because bindings are part of the state of a collection, and associate a URI that is *relative to that collection* with its target resource. No change to the bindings in Collection C1 is needed to make its children accessible using /COLLX/x.gif and /COLLX/y.jpg. +-------------------------+ | Root Collection | | (properties) | | bindings: | | coll1 COLLX | +-------------------------+ | / | / | / +------------------+ | Collection C1 | | (properties) | | bindings: | | x.gif y.jpg | +------------------+ | \ | \ | \ +-------------+ +-------------+ | Resource R1 | | Resource R2 | +-------------+ +-------------+ Figure 4 5.3 URI Mappings Created by BIND The set of URI mappings created by a successful BIND operation MUST be determined as follows: Slein et al. Page 9 Internet-Draft WebDAV Bindings August 1999 1. Start with an empty set of URLs, called U. 2. Take the Request-URI and remove path segments (and associated "/" characters) from the right until either (a) a non-WebDAV collection is found, or (b) the root, "/" is reached (i.e., no characters after the scheme and authority parts of the URL). This is the base URL B. 3. Add B, and all possible domain name variants of B (i.e., all other domain names which can be substituted for the domain name in B, and still retrieve the resource mapped to B), to URL set U. 4. Calculate the next path segment of the Request-URI, going from left to right, and call it S, which is bound to resource R. 5. For each member of URL set U, called Um, remove Um, then for every possible binding to R, create a new URL by adding the binding's path segment to Um, then add this new URL to U. 6. If there is no further path segment, then U has the complete set of URI mappings. Otherwise, go back to step 4. 5.4 Example: Generating the Set of URI Mappings Assume a server responds to two domain names, www.fuzz.com, and fuzz.com, and has a top level that is not WebDAV-aware, called A/. Below A/ is WebDAV-aware collection that is bound to both 1/ and one/. In collection one/ there is a resource called index.html. >> Request: BIND /A/1/info.html HTTP/1.1 Host: www.fuzz.com Destination: http://www.fuzz.com/A/one/index.html >> Response: HTTP/1.1 201 Created The set of all possible URI mappings to the resource identified by http://www.fuzz.com/A/one/index.html is calculated as follows: 1. U is empty. 2. The base URL, B, is http://www.fuzz.com/A/, since A/ is not WebDAV- aware. 3. Since there are two domain names for this server, the domain name variations of B are added to U, making U contain: http://www.fuzz.com/A/ and http://fuzz.com/A/. 4. (iteration 1) The next path segment of the Request-URI is 1/, which is bound to a collection resource, R. 5. (iteration 1) Since the collection resource R is bound to 1/ and one/, the value of U after the operation is: http://www.fuzz.com/A/1/, http://www.fuzz.com/A/one/, http://fuzz.com/A/1/, and http://fuzz.com/A/one/. 6. Go back to step 4, since there is one more path segment in the Request-URI. 4. (iteration 2) The next path segment of the Request-URI is info.html, which is bound to an HTML resource, R. 5. (iteration 2) Since the HTML resource is bound to info.html and index.html, the value of U after the operation is: Slein et al. Page 10 Internet-Draft WebDAV Bindings August 1999 http://www.fuzz.com/A/1/index.html, http://www.fuzz.com/A/1/info.html, http://www.fuzz.com/A/one/index.html, http://www.fuzz.com/A/one/info.html, http://fuzz.com/A/1/index.html, http://fuzz.com/A/1/info.html, http://fuzz.com/A/one/index.html, http://fuzz.com/A/one/info.html. 6. Since there are no further path segments in the Request-URI, U now has the complete set of URI mappings for the resource identified by the Destination header. 5.5 BIND Status Codes 201 (Created): The binding was successfully created. 400 (Bad Request): The client set an invalid value for the Destination header. 409 (Conflict): Several conditions may produce this response. The URI in the Destination header is not mapped to a resource. The request is attempting to create a binding in a collection that does not exist. The request is attempting to re-bind the top-level collection. 412 (Precondition Failed): The Overwrite header is "F", and a binding already exists for the request-URI. 5.6 Example: BIND >> Request: BIND /~whitehead/dav/spec08.txt HTTP/1.1 Host: www.ics.uci.edu Destination: http://www.ics.uci.edu/pub/i-d/draft-webdav-protocol-08.txt >> Response: HTTP/1.1 201 Created The server created a new binding, associating "spec08.txt" with the resource identified by the URL "http://www.ics.uci.edu/pub/i-d/draft- webdav-protocol-08.txt". Clients can now use the Request-URI, "http://www.ics.uci.edu/~whitehead/dav/spec08.txt", to submit requests to that resource. As part of this operation, the server added the binding "spec08.txt" to collection /~whitehead/dav/. 5.7 Example: BIND Conflict >> Request: BIND /press/prlogo.gif HTTP/1.1 Host: www.softcorp.com Destination: http://www.softcorp.com/logos/ >> Response: HTTP/1.1 409 Conflict Slein et al. Page 11 Internet-Draft WebDAV Bindings August 1999 The client requested the server to create a binding between "prlogo.gif" and the resource identified by the URL "http://www.softcorp.com/logos/". Since the Destination does end in a slash, while the Request-URI does not, the server failed the request, returning a 409 (Conflict) status code. 6 DELETE and Bindings The DELETE method requests that the server remove the binding between the resource identified by the Request-URI and the binding name, the last path segment of the Request-URI. The binding MUST be removed from its parent collection, identified by the Request-URI minus its trailing slash (if present) and final segment. The All-Bindings header may be used with DELETE to request that the server remove all bindings to the resource identified by the Request-URI. Once all bindings to the resource are removed, the server MAY reclaim system resources associated with the resource. If DELETE removes a binding to a resource, but there remain other bindings to that resource, the server MUST NOT reclaim system resources associated with the resource. Since DELETE as specified in [WebDAV] is not an atomic operation, it may happen that parts of the hierarchy under the request-URI cannot be deleted. In this case, the response is as described in [WebDAV]. [HTTP] states that "the DELETE method requests that the origin server delete the resource identified by the Request-URI." Because [HTTP] did not distinguish between bindings and resources, the intent of its definition of DELETE is unclear. We consider the definition presented here to be a clarification of the definition in [HTTP]. Section 8.6.1 of [WebDAV] states that during DELETE processing, a server "MUST remove any URI for the resource identified by the Request-URI from collections which contain it as a member." Servers that support bindings SHOULD NOT follow this requirement. 7 COPY and Bindings As defined in Section 8.8 of [WebDAV], COPY causes the resource identified by the Request-URI to be duplicated, and makes the new resource accessible using the URI specified in the Destination header. Upon successful completion of a COPY, a new binding is created between the last path segment of the Destination header (including trailing "/", if present), and the destination resource. The new binding is added to its parent collection, identified by the Destination header minus its trailing slash (if present) and final segment. As an example, suppose that a COPY is issued to URI 3 for resource R below (which is also mapped to URI 1 and URI 2), with the Destination header set to URIX. After successful completion of the COPY operation, resource R is duplicated to create resource R', and a new binding has been created which creates at least the URI mapping between URIX and the new resource (although other URI mappings may also have been created). Slein et al. Page 12 Internet-Draft WebDAV Bindings August 1999 URI 1 URI 2 URI 3 URIX | | | | | | | <---- URI Mappings ----> | | | | | +---------------------+ +------------------------+ | Resource R | | Resource R' | +---------------------+ +------------------------+ Figure 5 It might be thought that a COPY request with "Depth: 0" on a collection would duplicate its bindings, since bindings are part of the collection's state. This is not the case, however. The definition of Depth in [WebDAV] makes it clear that a "Depth: 0" request does not apply to a collection's members. Consequently, a COPY with "Depth: 0" does not duplicate the bindings contained by the collection. 8 MOVE and Bindings The MOVE method has the effect of creating a new binding to a resource (at the Destination), and removing an existing binding (at the Request- URI). The name of the new binding is the last path segment of the Destination header, and the new binding is added to its parent collection, identified by the Destination header minus its trailing slash (if present) and final segment. As an example, suppose that a MOVE is issued to URI 3 for resource R below (which is also mapped to URI 1 and URI 2), with the Destination header set to URIX. After successful completion of the MOVE operation, a new binding has been created which creates at least the URI mapping between URIX and resource R (although other URI mappings may also have been created). The binding corresponding to the final segment of URI 3 has been removed, which also causes the URI mapping between URI 3 and R to be removed. >> Before Request: URI 1 URI 2 URI 3 | | | | | | <---- URI Mappings | | | +---------------------+ | Resource R | +---------------------+ >> After Request: URI 1 URI 2 URIX | | | | | | <---- URI Mappings | | | +---------------------+ | Resource R | +---------------------+ Slein et al. Page 13 Internet-Draft WebDAV Bindings August 1999 Figure 6 Since MOVE as specified in [WebDAV] is not an atomic operation, it may happen that parts of the hierarchy under the request-URI cannot be moved. In this case, the response is as described in [WebDAV]. 8.1 Implementation Note In some situations, particularly when the destination is on a different server from the original resource, the server may implement MOVE by performing a COPY, performing some consistency maintenance on bindings and properties, and then performing a DELETE. In the end, all of the original bindings except the one corresponding to the Request-URI will be associated with the new resource. The binding corresponding to the URI in the Destination header will be associated with the new resource. And the original resource together with the binding corresponding to the Request-URI will have been deleted. This implementation is in accord with the definition of MOVE in [WebDAV], and is logically equivalent to the definition given above. The consistency maintenance processing that is required for this implementation is as follows: The DAV:creationdate property of the new resource SHOULD have the same value as the DAV:creationdate property of the old resource. The DAV:getlastmodified property of the new resource SHOULD have the same value as the DAV:getlastmodified property of the old resource. All URIs that were bound to the original resource except for the Request-URI MUST be bound instead to the new resource. Consider again the case where a MOVE is issued to URI 3 for resource R (which is also mapped to URI 1 and URI 2), with the Destination header set to URIX. Unlike the previous example, in this implementation, after successful completion of the MOVE operation, resource R has been duplicated as resource R'. The original bindings corresponding to URI 1 and URI2 are now associated with R'. The binding corresponding to the Request-URI (URI 3) has been removed. And a new binding has been created which creates at least the URI mapping between URIX and resource R'. Note that the server may reclaim the storage associated with resource R once the MOVE operation has finished. >> Before Request: URI 1 URI 2 URI 3 | | | | | | <---- URI Mappings | | | +---------------------+ | Resource R | +---------------------+ >> After Request: Slein et al. Page 14 Internet-Draft WebDAV Bindings August 1999 URI1 URI2 --------------------------------- URIX | | | ----------------------------------------- | | | | | +---------------------+ +------------------------+ | Resource R | | Resource R' | +---------------------+ +------------------------+ Figure 7 9 LOCK and UNLOCK Bindings do not affect the semantics of locks, as specified in [WebDAV]. Specifically, the requirement in section 8.10.3 that "a LOCK request on a resource MUST NOT succeed if it can not be honored by all the URIs through which the resource is accessible" still holds. The LOCK method locks the resource, and a lock is visible via all URIs mapped to that resource. Similarly, a successful UNLOCK issued via any URI mapping to a resource removes the lock from the resource, and this lock removal is visible via all URI mappings. When a resource is locked, the lock owner expects to be able to access the resource -- using the same Request-URI that he used to lock the resource -- for as long as he holds the lock. This would not be possible if another user could move or delete any of the collections corresponding to segments of the request-URI. Consequently, for the duration of a lock, it MUST NOT be possible for a principal other than the lock owner to make a locked resource inaccessible via the URI mapping used to lock the resource. Only the lock owner can move or delete any of the collections corresponding to segments of the Request-URI. This restriction does not prevent others from modifying those collections, by adding members to them, removing members from them, or changing their property values. For example, if a user locks /plants/herbs/rosemary.html, it is not possible for another user to move /plants/herbs/ to /plants/flowering/herbs/, or to completely delete /plants/herbs/, though it is possible this delete operation may succeed in deleting everything except for /plants/herbs/rosemary.html and /plants/herbs/. 10 Bindings and Other Methods This section describes the interaction of bindings with those HTTP methods not yet explicitly discussed. The semantics of the methods GET, HEAD, PUT, POST and OPTIONS are specified in [HTTP]. The semantics of PROPFIND, PROPPATCH, and MKCOL are specified in [WebDAV]. For most of these methods, no new complexities are introduced by allowing explicit creation of multiple bindings to the same resource. For the access operations (GET, HEAD, OPTIONS, and PROPFIND), it is simply the case that no matter which URI mapping to a given resource is used as the Request-URI, the response is mediated by that same resource. The responses may, however, vary depending upon which Request-URI was used. For example, the response to a GET request may contain the Slein et al. Page 15 Internet-Draft WebDAV Bindings August 1999 Request-URI in its entity. The same is true for POST. No matter which URI mapping to a given resource is used as the Request-URI, the response is mediated by that same resource. The changes made on the server and the responses may, however, vary depending upon which Request-URI was used. If the Request-URI of a PUT identifies an existing resource, then a PUT via one URI mapping to this resource MUST produce the same result as a PUT with the same headers and request entity body via any other URI mapping to the same resource. The change made by a PUT via one URI mapping MUST affect the resource that generates the GET response for all URI mappings to the same resource. A PROPPATCH through one URI mapping to a resource MUST produce the same changes to its properties as the same PROPPATCH request through a different URI mapping to the same resource. As specified in [WebDAV], MKCOL cannot overwrite an existing resource. MKCOL through any URI mapping to an existing resource must fail. The semantics of MKREF are specified in Section nn of [RR]. A MKREF through one URI mapping to a resource MUST produce the same result as a MKREF with the same headers through any other URI mapping to the same resource. By default, it overwrites the resource with a new redirect reference. The semantics of ORDERPATCH are specified in Section nn of [OC]. An ORDERPATCH through one URI mapping to a collection MUST produce the same changes to its ordering as the same ORDERPATCH request through any other URI mapping to the same collection. 11 Discovering the Bindings to a Resource An OPTIONAL DAV:bindings property on a resource provides a list of the bindings that associate URI segments with that resource. By retrieving this property, a client can discover the bindings that point to the resource and the collections that contain bindings to the resource. As for all DAV: properties, this specification is silent as to how the DAV:bindings property is implemented on the server. Rationale: A number of scenarios require clients to navigate from a resource to the bindings that point to it, and to the collections that contain those bindings. This capability is particularly important for Document Management Systems. Their clients may need to determine, for any object in the DMS, what collections contain bindings to that object. This information can be used for upward navigation through a hierarchy or to discover related documents in other collections. Risks: When deciding whether to support the DAV:bindings property, server implementers / administrators should balance the benefits it provides against the cost of maintaining the property and the security risks enumerated in Sections 17.4 and 17.5. 12 Headers Slein et al. Page 16 Internet-Draft WebDAV Bindings August 1999 12.1 All-Bindings Request Header All-Bindings = "All-Bindings" ":" The All-Bindings request header may be used with DELETE requests to instruct the server to remove all bindings to the resource identified by the Request-URI. 13 Status Codes 13.1 506 Loop Detected The 506 (Loop Detected) status code indicates that the server detected an infinite loop while processing a request with "Depth: infinity". 14 Properties 14.1 bindings Property Name: bindings Namespace: DAV: Purpose: Enables clients to discover, for any resource, what bindings point to it and what collections contain those bindings. This is an optional property. If present, it is a read-only property, maintained by the server. Value: List of href / segment pairs for all of the bindings that associate URI segments with the resource. The href is an absolute URI for one URI mapping of the collection containing the binding. Since there may be multiple URI mappings for this collection, it is necessary to select one (preferably the URI mapping contained in the Request-URI of the BIND request) for use in the DAV:bindings property. The segment is the URI segment that identifies the binding within that collection. 15 XML Elements 15.1 segment XML Element Name: segment Namespace: DAV: Purpose: The segment that names a binding, used in the DAV:bindings property. Value: segment ; as defined in section 3.3 of [URI]. 16 Capability Discovery Sections 9.1 and 15 of [WebDAV] describe the use of compliance classes with the DAV header in responses to OPTIONS, to indicate which parts of the Web Distributed Authoring protocols the resource supports. This Slein et al. Page 17 Internet-Draft WebDAV Bindings August 1999 specification defines an OPTIONAL extension to [WebDAV]. It defines a new compliance class, called bindings, for use with the DAV header in responses to OPTIONS requests. If a resource does support bindings, its response to an OPTIONS request MUST indicate that it does, by listing the new BIND method as one it supports, and by listing the new bindings compliance class in the DAV header. When responding to an OPTIONS request, any type of resource can include bindings in the value of the DAV header. Doing so indicates that the server permits a binding at the request URI. 16.1 Example: Discovery of Support for Bindings >> Request: OPTIONS /somecollection/someresource HTTP/1.1 HOST: somehost.org >> Response: HTTP/1.1 200 OK Date: Tue, 20 Jan 1998 20:52:29 GMT Connection: close Accept-Ranges: none Allow: OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, COPY, MOVE, MKCOL, PROPFIND, PROPPATCH, LOCK, UNLOCK, BIND Public: OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, COPY, MOVE, MKCOL, PROPFIND, PROPPATCH, LOCK, UNLOCK, BIND, MKREF, ORDERPATCH DAV: 1, 2, bindings The DAV header in the response indicates that the resource /somecollection/someresource is level 1 and level 2 compliant, as defined in [WebDAV]. In addition, /somecollection/someresource supports bindings. The Allow header indicates that BIND requests can be submitted to /somecollection/someresource. The Public header shows that other Request-URIs on the server support additional methods. 17 Security Considerations This section is provided to make WebDAV applications aware of the security implications of this protocol. All of the security considerations of HTTP/1.1 and the WebDAV Distributed Authoring Protocol specification also apply to this protocol specification. In addition, bindings introduce several new security concerns and increase the risk of some existing threats. These issues are detailed below. 17.1 Privacy Concerns In a context where cross-server bindings are supported, creating bindings on a trusted server may make it possible for a hostile agent to induce users to send private information to a target on a different server. Slein et al. Page 18 Internet-Draft WebDAV Bindings August 1999 17.2 Redirect Loops Although redirect loops were already possible in HTTP 1.1, the introduction of the BIND method creates a new avenue for clients to create loops accidentally or maliciously. If the binding and its target are on the same server, the server may be able to detect BIND requests that would create loops. Servers are required to detect loops that are caused by bindings to collections during the processing of any requests with "Depth: infinity". 17.3 Bindings, and Denial of Service Denial of service attacks were already possible by posting URLs that were intended for limited use at heavily used Web sites. The introduction of BIND creates a new avenue for similar denial of service attacks. If cross-server bindings are supported, clients can now create bindings at heavily used sites to target locations that were not designed for heavy usage. 17.4 Private Locations May Be Revealed If the DAV:bindings property is maintained on a resource, the owners of the bindings risk revealing private locations. The directory structures where bindings are located are available to anyone who has access to the DAV:bindings property on the resource. Moving a binding may reveal its new location to anyone with access to DAV:bindings on its resource. 17.5 DAV:bindings and Denial of Service If the server maintains the DAV:bindings property in response to bindings created in other administrative domains, it is exposed to hostile attempts to make it devote resources to adding bindings to the list. 18 Internationalization Considerations This specification follows the practices of [WebDAV] in encoding all human-readable content using XML [XML] and in the treatment of names. Consequently, this specification complies with the IETF Character Set Policy [Alvestrand]. WebDAV applications MUST support the character set tagging, character set encoding, and the language tagging functionality of the XML specification. This constraint ensures that the human-readable content of this specification complies with [Alvestrand]. As in [WebDAV}, names in this specification fall into three categories: names of protocol elements such as methods and headers, names of XML elements, and names of properties. Naming of protocol elements follows the precedent of HTTP, using English names encoded in USASCII for methods and headers. The names of XML elements used in this specification are English names encoded in UTF-8. For error reporting, [WebDAV] follows the convention of HTTP/1.1 status codes, including with each status code a short, English description of Slein et al. Page 19 Internet-Draft WebDAV Bindings August 1999 the code (e.g., 423 Locked). Internationalized applications will ignore this message, and display an appropriate message in the user's language and character set. For rationales for these decisions and advice for application implementors, see [WebDAV]. 19 IANA Considerations This document uses the namespaces defined by [WebDAV] for properties and XML elements. All other IANA considerations mentioned in [WebDAV] also apply to this document. 20 Copyright To be supplied by the RFC Editor. 21 Intellectual Property To be supplied by the RFC Editor. 22 Acknowledgements This draft has benefited from thoughtful discussion by Jim Amsden, Steve Carter, Ken Coar, Ellis Cohen, Dan Connolly, Bruce Cragun, Spencer Dawkins, Mark Day, Rajiv Dulepet, David Durand, Roy Fielding, Yaron Goland, Fred Hitt, Alex Hopmann, Marcus Jager, Chris Kaler, Manoj Kasichainula, Rohit Khare, Daniel LaLiberte, Steve Martin, Larry Masinter, Jeff McAffer, Surendra Koduru Reddy, Max Rible, Sam Ruby, Bradley Sergeant, Nick Shelness, John Stracke, John Tigue, John Turner, Kevin Wiggen, and others. 23 References [URI] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax." RFC 2396. MIT/LCS, U.C. Irvine, Xerox. August, 1998. [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels." RFC 2119, BCP 14. Harvard University. March, 1997. [XML] T. Bray, J. Paoli, C.M. Sperberg-McQueen, "Extensible Markup Language (XML)." World Wide Web Consortium Recommendation REC-xml- 19980210. http://www.w3.org/TR/1998/REC-xml-19980210. [HTTP] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1." RFC 2616. UC Irvine, Compaq, W3C, Xerox, Microsoft. June, 1999. [WebDAV] Y. Y. Goland, E. J. Whitehead, Jr., A. Faizi, S. R. Carter, D. Jensen, "HTTP Extensions for Distributed Authoring - WebDAV." RFC 2518. Microsoft, U.C. Irvine, Netscape, Novell. February, 1999. [OC] J. Slein, E.J. Whitehead Jr., J. Davis, G. Clemm, C. Fay, J. Crawford, T. Chihaya, "WebDAV Ordered Collections." Internet Draft (work Slein et al. Page 20 Internet-Draft WebDAV Bindings August 1999 in progress) draft-ietf-webdav-ordering-protocol-00. Xerox, UC Irvine, CourseNet, Rational, FileNet, IBM, DataChannel. August, 1999. [RR] J. Slein, E.J. Whitehead Jr., J. Davis, G. Clemm, C. Fay, J. Crawford, T. Chihaya, "WebDAV Redirect References." Internet Draft (work in progress) draft-ietf-webdav-redirectref-protocol-00. Xerox, UC Irvine, CourseNet, Rational, FileNet, IBM, DataChannel. August, 1999. 24 Authors' Addresses J. Slein Xerox Corporation 800 Phillips Road, 105-50C Webster, NY 14580 Email: jslein@crt.xerox.com E. J. Whitehead, Jr. Dept. of Information and Computer Science University of California, Irvine Irvine, CA 92697-3425 Email: ejw@ics.uci.edu J. Davis CourseNet Systems 170 Capp Street San Francisco, CA 94110 Email: jrd3@alum.mit.edu G. Clemm Rational Software Corporation 20 Maguire Road Lexington, MA 02173-3104 Email: gclemm@rational.com C. Fay FileNet Corporation 3565 Harbor Boulevard Costa Mesa, CA 92626-1420 Email: cfay@filenet.com J. Crawford IBM Email: ccjason@us.ibm.com T. Chihaya DataChannel, Inc. 155 108th Ave. N.E., Suite 400 Bellevue, WA 98004 Email: Tyson@DataChannel.com 25 Appendices 25.1 Appendix 1: Extensions to the WebDAV Document Type Definition Slein et al. Page 21 Internet-Draft WebDAV Bindings August 1999 Expires February 20, 2000 Slein et al. Page 22